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Nutrient Fate in the Full MAnure PHosphorus EXtraction (MAPHEX) System, and Design of a Simplified System (MAPHEX Lite)
Clinton Church1,*, Alexander N. Hristov2, Peter J. A. Kleinman1, Sarah K. Fishel1, Michael R. Reiner1, Ray B. Bryant1
Published in Applied Engineering in Agriculture 39(3): 339-346 (doi: 10.13031/aea.15365). 2023 American Society of Agricultural and Biological Engineers.
1Pasture Systems and Watershed Management Unit, USDA ARS, University Park, Pennsylvania, USA.
2College of Agricultural Sciences, Pennsylvania State University, Harrisburg, Pennsylvania, USA.
*Correspondence: clinton.church@usda.gov
The authors have paid for open access for this article. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License https://creative commons.org/licenses/by-nc-nd/4.0/
Submitted for review on 23 September 2022 as manuscript number NRES 15365; approved for publication as a Research Article by Associate Editor Dr. Xiaoyu (Iris) Feng and Community Editor Dr. Kati Migliaccio of the Natural Resources & Environmental Systems Community of ASABE on 16 May 2023.
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Highlights
Abstract. The relatively recent concept of the manureshed highlights the problem of the broken nutrient cycle in modern animal agriculture and the low nitrogen:phosphorus ratio in manure relative to crop requirement that results in P accumulation in soils near source areas. One solution to avoid P accumulation is to transport the manure to soils with a deficit of P, but liquid manure’s bulkiness and low nutrient density present challenges for transport over great distances. While the full MAnure PHosphorus EXtraction (MAPHEX) System has shown to be capable of removing greater than 90% of the P from liquid manures while leaving much of the N in the liquid fraction for use on the farm, other nutrients present in manures in lesser amounts than N and P have not been reported on. This study indicates that both the full MAPHEX System and a newly designed MAPHEX Lite System, that not only conserves more N but is more efficient and less costly, are highly efficient at extracting and concentrating most nutrients in solid form while leaving most of the N and K in the liquid phase for beneficial use by the farmer near the manure source. Therefore, it seems clear that both Systems, and the components they include have the potential to play a significant role in manureshed management.
The relatively recent concept of the manureshed shines a spotlight on the problem of the broken nutrient cycle in modern animal agriculture and provides a framework for addressing the issue (Spiegal et al., 2020). Manureshed analyses also highlight the low nitrogen:phosphorus (N:P) ratio in manure relative to crop requirement that results in legacy P accumulation in soils (Sabo et al., 2021) and greater transport distances required for P-based nutrient management compared to N-based management. Transporting nutrients from areas of concentrated livestock and poultry production, known as source areas, to croplands where nutrients are needed, known as sink areas, is a lofty goal, but the bulkiness of manure and low nutrient density present challenges for transporting manure over great distances. Whereas relatively dry poultry litter lends itself to transport (Bryant et al., 2021), liquid dairy manures characteristic of dairy production in the eastern United States (Dell et al., 2022) and liquid swine manures are especially difficult to transport large distances (Meinen et al., 2022), and bulk transport of raw, liquid manure is also potentially damaging to roads and infrastructure.
The need to treat manure to concentrate nutrients so they can be transported more efficiently is not only recognized under the manureshed framework but methods for treating manure and concentrating nutrients have received considerable attention in recent decades. Several chemical manure treatments have shown promise for removing P from manures and wastewaters, including precipitation as calcium or magnesium phosphates (Güngor and Karthikeyan, 2008), precipitation as struvite (Bowers and Westerman, 2005; Yilmazel and Demirer, 2013, Harrison et al., 2022), or precipitation as newberyite (Vanotti et al., 2017, 2018). While recovery and concentration of P by these methods has been shown to be highly effective (95% to 98% removal) on manures containing less than 2% total solids, none of them have been shown to be effective on raw or agitated manures which are typically 8% to 12% total solids.
The MAPHEX system was specifically designed to address the problem of excess P on the farm by removing and concentrating P from high solids (8% to 12% total solids) raw and agitated manures so it can be transported more easily to distant areas of the farm where P is needed or exported from the farm. Meanwhile, N is left behind in the liquid phase to be used nearer the manure source (Church et al., 2016, 2017, 2018, 2020). The chemical mechanism employed to remove dissolved P is a simple sorption mechanism to iron oxyhydroxides, followed by filtration of the solids formed. Phosphorus removal efficiencies for P are greater than 90%, and the P is concentrated in a stackable solid form (70% to 75% moisture) that is ideal for composting, sale, or economically transporting to lands that need P (Church et al., 2016, 2017, 2018, 2020).
While the MAPHEX System works well to remove P and concentrate it in a solid fraction suitable for transport off the farm or to areas further from manure storage facilities in need of P and leave much of the N in the liquid fraction for use on the farm, other nutrients used by crops and present in manures in lesser amounts than N and P have been of lesser concern. However, with the cost of fertilizers almost doubling in the year prior to the planting season of 2022 (Quinn, 2022), the fate of potassium and other nutrients passing through the MAPHEX system has taken on greater economic concern. The objective of this study is to determine what proportions of these other nutrients are captured in the solid fraction that is destined to leave the farm and what proportions remain in the liquid fraction for use on the farm. A more complete understanding the fate of nutrients other than P contributed to development of a new System, referred to as MAPHEX Lite, that not only conserves more N but is more efficient and less costly.
Materials and Methods
The MAPHEX System
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| Figure 1. The full-scale MAPHEX System. |
All manure treatment systems require or benefit from some level of liquid/solid separation prior to chemical treatment. This physical pre-treatment facilitates chemical mixing, lowers the buffering capacity of the manure to allow for pH adjustments if needed, and allows heavier solid precipitates formed by the chemical treatment process to be readily separated from the liquid phase. The full MAPHEX System was used in this study (fig. 1) and consists of components to perform: a) an initial liquid-solid separation step broken into two stages, b) chemical treatment, and c) a final liquid-solid separation step using an AutoVac®, as well as ancillary components (pumps, piping, etc.) to feed the major components (Church et al., 2016, 2017, 2018, 2020). Manure slurry is pumped through the system where manure particulate and P is removed at each of the liquid-solid separation steps.
- Initial Liquid-Solid Separation. The initial liquid-solid separation can be done in either one or two stages, depending upon the intended use of the solids and the farm’s needs, but it is critical that the overall process leaves only particles smaller than 30 µm diameter in the liquid effluent for the following chemical treatment to be effective. In this study the full-scale MAPHEX System used two stages of liquid-solid separation, removing the bulk solids (<1 mm) from dairy manure with either two auger presses (Neptune Enterprises, Richland Center, Wis.) or an existing screw press, and then removing the medium sized particles (down to about 25 µm diameter) with a decanter centrifuge (Sharples P-3400, Alfa Laval, Lund, Sweden). The reason for using two stages is so that bulk solids removed by the auger press, which are low in P relative to the other two solids generated by the system, can be left behind on the farm to be composted and used as bedding material therefore, reducing the input costs for the farm. For farms that do not use the compost for bedding, the bulk solids could be blended with the other solids from the system and sold as fertilizer after composting or more economically spread on fields a greater distance from the dairy.
- Chemical Treatment. After the initial liquid-solid separation step, the liquids were subjected to chemical treatment [3.0 g l-1 Fe2(SO4)3] to coagulate the particles and to transform dissolved P (primarily orthophosphate) into a solid form that can be removed in the final liquid solid-separation step. This treatment can either be done in batch mode in tanks or could be accomplished by injecting the chemicals into the liquid stream.
- Final Liquid-Solid Separation. The final liquid-solid separation removes the P converted in the chemical treatment step along with the fine solids (those between 0.5 and 25 µm diameter). The only existing technology we identified that efficiently performed this step while simultaneously yielding a dry stackable solid was an AutoVac® AV660 unit manufactured by ALAR Engineering Corporation (Mokena, Ill.). This unit uses a rotating drum coated with diatomaceous earth (DE) that has pore spaces of approximately 0.5 µm and is designed so that the filtration surface is constantly renewed. A vacuum (61 to 85 kPa) is pulled on the drum to draw liquids through the diatomaceous earth filter.
Manures Tested
In order to demonstrate the versatility of the MAPHEX System, we purposely chose to use farms that varied widely in herd size, bedding type used, and in the existing methods of manure treatment and handling to represent manures common throughout the dairy industry. Herd size ranged from 90 to 5500 milking cows, bedding on the farms was either fine sized fresh sawdust or composted manure, and existing manure treatment on the farms ranged from storage only (the 90- and 150-cow farms), to bulk liquid-solid separation aimed at bedding recovery (the 2700-cow farm), to anaerobic digestion followed by bulk liquid-solid separation for bedding recovery (the 5500-cow farm). In cases where the existing technology performed a similar function as components of the MAPHEX System (such as bulk liquid-solid separation), we bypassed that component of the MAPHEX System, and fed the effluent from the farms treatment technology directly to the next component of the MAPHEX System. A brief explanation of the manure treatment systems at each dairy are provided below. Raw manure characteristics are shown in table 1.
90-Cow Dairy
The MAPHEX System was used to treat manure on a small dairy farm in eastern Pennsylvania with 90 lactating Holstein dairy cows housed in a free stall barn with slots in the floor allowing manure to fall directly into the manure storage pit below the barn. Manure from an attached free stall barn housing 40 non-lactating cows is also regularly scraped into the same pit. All free stalls were regularly bedded with fine sawdust. Dairy manure was pumped directly from the manure storage pit into the two auger presses and the effluent fed to subsequent equipment of the MAPHEX System.
| Table 1. Raw manure characteristics. | ||||||||||||
| Dairy Farm | % | Mean (Standard Error) | ||||||||||
| n[a] | Dry Matter | P (mg kg-1) | N (mg kg-1) | K (mg kg-1) | Al (mg kg-1) | Ca (mg kg-1) | Zn (mg kg-1) | Mn (mg kg-1) | Mg (mg kg-1) | Fe (mg kg-1) | S (mg kg-1) | |
| 90 cow | 8 | 14.1 (2) | 282 (30) | 3,892 (NA) | 1,436 (41) | 14 (2) | 990 (102) | 12 (2) | 11 (1) | 394 (31) | 37 (4) | 234 (17) |
| 150 cow | 3 | 5.2 (1) | 372 (3) | 3,720 (105) | 1,528 (13) | 38 (1) | 1,302 (24) | 11 (0.1) | 15 (0.2) | 505 (2) | 71 (3) | 224 (5) |
| 2700 cow | 2 | 5.2 (1) | 333 (5) | 3,211 (113) | 1,151 (33) | 35 (3) | 5,891 (712) | 19 (0.3) | 17 (1) | 521 (13) | 59 (3) | 233 (1) |
| 5500 cow | 4 | 3.6 (1) | 618 (31) | 3,003 (283) | 1,210 (127) | 14 (2) | 1,302 (102) | 17 (1) | 10 (1) | 631 (41) | 34 (4) | 340 (40) |
[a] n = number of samples measured. Note: Dry matter and P data were previously reported in Church et al. (2018), but are included here for comparison and discussion. | ||||||||||||
150-Cow Dairy
Manure slurry was obtained from a small dairy farm in central Pennsylvania with 150 lactating Holstein dairy cows housed in a free stall barn that was regularly bedded with coarse wood chips. Dairy manure was scraped daily into a holding pit and transferred to an above ground slurry holding tank. Manure from the open slurry holding tank was agitated prior to collection in a 8800 L tanker for processing. Manure was pumped directly from the recirculating tanker into the two auger presses and effluent fed to subsequent equipment of the MAPHEX System.
2700-Cow Dairy
The medium sized dairy on which we treated manure was located in Central Pennsylvania and had approximately 2700 lactating Holstein dairy cows housed in free stall barns. Manure from the barns was flushed, with recycled effluent from the screw press process, into an open holding tank where it was fed continually to an on-farm screw press for generation of bedding material and liquid-solid separation. Since the screw press performs much the same function as our auger presses, manure from the holding tank after the screw press was pumped directly into the centrifuge of the MAPHEX system for testing. One major difference in this dairy’s manure treatment compared to the other two dairies is that they amend their solids from the screw press with lime for bacterial control during composting for use as bedding. The added lime results in manure slurry with an elevated alkalinity (Church et al., 2018) and Ca (table 1) when portions of those amended solids inevitably end up back in the manure slurry.
5500-Cow Dairy
The largest dairy from which we used manure was located in Eastern Wisconsin and had approximately 5500 lactating Holstein dairy cows housed in free stall barns. Manure from the barns was scraped and flushed into a holding pit before being fed into a large anaerobic digester. Manure leaving the digester was fed to an on-farm screw press for solids removal and fed down a pipe to a storage lagoon. Manure from this pipe was pumped directly into the centrifuge of the MAPHEX system for testing.
Sample Collection/Preservation and Laboratory Analysis
Sample Collection Preservation and Characterization
Samples were taken of the raw dairy manure slurry, and of effluents and solid rejects from the liquid-solid separation steps (directly from the MAPHEX System or from the analogous on-farm equipment) over the duration of manure processing to test for solids content and nutrient concentrations (discussed below). Raw manure slurry samples were collected from a well-stirred tank or lagoon, while effluent was collected at intervals ranging between 30 min and 1 h (dependent upon the planned overall processing time) during the entire operation of the MAPHEX System. Solid samples were collected each time the solids needed to be removed and were comprised of a composite of 7 sub-samples taken randomly throughout the pile. All samples were stored at 4°C until analyzed or used in laboratory-scale testing. Dry matter content was determined by oven drying solids and effluents at 110°C for 16 h (Peters, 2003).
EPA 3050b Metals Determination
Samples were subjected to EPA 3050B extraction for metals and Kjeldahl digest N (TKN) analysis. Briefly, manure slurries and solids were extracted with aqua regia and hydrogen peroxide following a modified EPA Standard Method 3050B (Kimbrough and Wakakuwa, 1989). Liquid effluents were treated as water samples using 10 mL of sample. Solids were digested as received, such that the sample size contained 0.5 g of dry solid material. Following dilution to final volumes and filtration (Whatman No. 1), analysis was then performed on all extracts using an inductively coupled optical emission spectrophotometer (ICP-OES, Varian).
Kjeldahl Digest N Determination
Extractions were also carried out for Total Kjeldahl N (TKN) determination (Gallaher et al., 1976; Peters, 2003). Briefly, a 0.5 g solids-weight of material was weighed into digestion tubes and a CT-37 Kjeldahl digest tablet and 7 mL of concentrated sulfuric acid were added. The sample was then digested at 375°C for 2 h and diluted to final volume. After filtration through Whatman No. 1 filters, N analysis was then performed by Quick Chem Method 10-107-06-2-H (Lachat Instruments, 2003).
Hydrogen Sulfide Measurement
Hydrogen sulfide (H2S) was measured using a ventis max4 manure monitor (Industrial Scientific Corporation, Pittsburgh, Pa.). While this instrument is primarily a hazardous gas detector rather than an analytical quality instrument, it gives relative measurements of hazardous gasses.
Data Analysis
Nutrient removal efficiencies based on values obtained from analyses above were determined. Reported means and standard errors were calculated by an Excel spreadsheet.
| Table 2. Separated manure solids, N, P, and K. | ||||||||
| % | Mean (Standard Error) | |||||||
| Dairy Farm | n[a] | Dry Matter | P (mg kg-1) | N (mg kg-1) | K (mg kg-1) | |||
| 90 cow | Auger effluent Centrifuge effluent AutoVac effluent | 7 10 10 | 7.1 (1) 3.5 (0.4) 1.4 (0.2) | 336 (27) 198 (13) 25 (4) | . . 1,540 (56) | 1,469 (46) 1,421 (10) 934 (123) | ||
| Auger solids Centrifuge solids AutoVac solids | 5 8 5 | 20.0 (1) 30.1 (5) 40.5 (1) | 1,041 (55) 450 (20) 878 (42) | 4,770 (304) 6,023 (NA) 4,075 (116) | 1,369 (78) 1,455 (53) 1,114 (37) | |||
| 150 cow | Auger effluent Centrifuge effluent AutoVac effluent | 10 7 14 | 5.0 (0.2) 2.3 (0.2) 0.3(0.1) | 373 (3) 219 (2) 28 (2) | . . . | 1,548 (10) 1,561 (3) 1,191 (74) | ||
| Auger solids Centrifuge solids AutoVac solids | 5 5 8 | 25.5 (2) 31.1 (0.4) 40.8 (1) | 495 (17) 1,847 (47) 975 (16) | . 5,518 (162) . | 1,551 (28) 1,775 (19) 1,390 (44) | |||
| 2700 cow | Screw press effluent Centrifuge effluent AutoVac effluent | 6 3 12 | 3.8 (0.4) 3.0 (0.5) 0.7 (0.1) | 279 (2) 211 (9) 41 (5) | . 2,554 (97) 1,349 (121) | 1,049 (5) 982 (28) 756 (64) | ||
| Auger solids Centrifuge solids AutoVac solids | 2 2 7 | 32.8 (3) 29.3 (2) 38.8 (1) | 555 (128) 1,512 (543) 744 (35) | . 5,581 (198) 4,747 (402) | 1,287 (115) 1,499 (383) 792 (42) | |||
| 5500 cow | Screw press effluent Centrifuge effluent AutoVac effluent | 7 9 13 | 3.8 (0.2) 2.7 (0.1) 1.4 (0.1) | 520 (25) 328 (11) 88 (6) | 3,113 (28) . 1,844 (79) | 1,354 (35) 1,371 (12) 1,240 (60) | ||
| Centrifuge solids AutoVac solids | 10 12 | 20.5 (1) 38.4 (1) | 3,288 (213) 1,688 (108) | 7,366 (752) 7,317 (524) | 1,745 (53) 1,273 (39) | |||
[a] n = number of samples measured. Note: Dry matter and P data in this table were previously reported in Church et al. (2018), but are included here for comparison. | ||||||||
Results and Discussion
Solids and Removal
As reported previously (Church et al., 2018), the full MAPHEX System was effective at removing total solids from a wide variety of raw manures, leaving 0.3% – 1.4% solids in the final effluent (table 2). In all cases, solids removed from the manure slurry by various machinery were stackable, containing between 70% and 75% moisture. Furthermore, the pore space (about 0.7 µm) of the AutoVac® ensured that the final effluent could readily be pumped or applied to fields through sprinkler systems without causing clogging.
We further found the full MAPHEX System was highly effective at removing P, ranging from 88% to 92% removal for all farms tested (tables 2 and 4). A closer examination of the data however (tables 1 and 2), highlights the difference in P removal efficiency between the auger presses and a screw press. On the 2700- and 5500-cow dairies, which had an existing screw press, screw press effluent concentration was 16.2% and 15.9% lower, respectively. By contrast, while on a mass basis the auger press removed 15% and 9% respectively, for the 90- and 150-cow dairies (Church et al., 2018), the effluent concentration was actually higher in the auger press effluent than in the raw manure (tables 1 and 2) because the solids removed had a much lower P concentration than the liquid fraction. We believe that there were two reasons for this: 1) the two smaller farms bedded on fresh sawdust and wood chips, which likely contained a lower concentration of P compared to the manure compost bedding of the larger farms, and 2) the much smaller slot size in the screw press (1 mm) compared to the two auger presses (3.2 and 4.8 mm) retained more small particle sized solids. In general, we have observed that, with manure solids, smaller particle fractions contain higher concentrations of P than larger particle sized fractions (Church et al., 2016, 2017, 2018, 2020).
As the previous studies on the MAPHEX System were focused on P removal, N concentration measurements were primarily focused on solids fractions to assess their fertilizer value. Calculations on a mass basis comparing the solids to the raw manure indicated that only 8% of the N was accounted for in the solids, suggesting that N was primarily retained in the liquid phase (Church et al., 2018). However, only a limited number of effluent samples were analyzed for N. This limited data (tables 2 and 4) suggests that overall N removal for the full System ranged from 39% to 68%. Furthermore, data from the 2700-cow dairy, where both centrifuge and AutoVac® effluent were analyzed, shows that the combination of the screw press and centrifuge removed only 20% of total N, while the AutoVac® removed 38%. The most likely explanation for the discrepancy between values based on solids and effluent concentrations is the loss of ammonia to the atmosphere. For the screw press/centrifuge combination, it is most likely agitation in the high energy environment of the centrifuge that is the culprit, as we have also measured H2S gaseous emissions coming from the centrifuge. In the case of the AutoVac®, ammonia loss is most likely attributable to the vacuum pulled on the drum to draw liquids through the DE filter, as we have also measured H2S emissions from the AutoVac® exhaust.
The data shows that potassium is largely a dissolved constituent, remaining in the liquid phase of the manure, with removal efficiencies in the effluent compared to raw manure ranging from 25% to 35% (table 4), with the exception of the farm with the anaerobic digester, which actually had slightly higher mean concentrations of K in the final effluent than in the raw manure (tables 1 and 2). In this case, the higher concentrations observed compared to raw manure may be due to volume changes from removing low concentration solids like with P above, but also may be due to addition of K to the anaerobic digester, as details of the digester operation were not available to us. The data suggests however, that the anaerobic digestion process serves to convert particles containing K into smaller particles or into dissolved constituents. This is also borne out by the K removal efficiencies by the combination of the screw/auger press and centrifuge alone, which ranged considerably lower, at -13% to 15%.
| Table 3. Separated manure micronutrients. | |||||||||
| Mean (Standard Error) | |||||||||
| Dairy Farm | n[a] | Al (mg kg-1) | Ca (mg kg-1) | Zn (mg kg-1) | Mn (mg kg-1) | Mg (mg kg-1) | Fe (mg kg-1) | S (mg kg-1) | |
| 90 cow | Auger effluent Centrifuge effluent AutoVac effluent | 7 10 10 | 17 (2) 7.4 (1) 0.5 (0.2) | 1,144 (85) 707 (32) 288 (37) | 14 (1) 9 (1) 1.9 (0.3) | 13 (1) 7 (1) -0.4 (0.3) | 436 (28) 318 (5) 158 (19) | 45 (4) 32 (9) 16 (3) | 245 (13) 183 (9) 110 (15) |
| Auger solids Centrifuge solids AutoVac solids | 5 8 5 | 84 (4) 28 (1) 73.5 (2) | 3,436 (184) 1,841 (64) 2,723 (77) | 33 (1) 21 (1) 38 (2) | 42 (3) 18 (1) 34 (1) | 1,015 (69) 535 (22) 679 (26) | 263 (30) 120 (12) 478 (15) | 594 (33) 512 (24) 457 (18) | |
| 150 cow | Auger effluent Centrifuge effluent AutoVac effluent | 10 7 14 | 39 (1) 26 (1) 1.4 (0.2) | 1,320 (12) 883 (4) 193 (8) | 11 (0.1) 11 (0.4) 1.5 (0.1) | 15 (0.1) 11 (0.1) 0.2 (0.2) | 511 (3) 385 (2) 240 (15) | 83 (8) 45 (1) 29 (3) | 218 (3) 176 (1) 327 (33) |
| Auger solids Centrifuge solids AutoVac solids | 5 5 8 | 59 (3) 171 (5) 163 (4) | 2,665 (153) 5,732 (110) 4593 (75) | 16 (1) 23 (0.4) 48 (1) | 23 (1) 60 (1) 62 (1) | 658 (22) 1,800 (43) 718 (22) | 205 (62) 376 (12) 2,547 (302) | 493 (35) 697 (7) 872 (33) | |
| 2700 cow | Press effluent Cent. effluent AutoVac effluent | 6 3 12 | 44 (9) 28 (1) 14 (1.1) | 4,797 (100) 2,276 (188) 387 (36) | 16 (0.1) 14 (1) 2.9 (0.3) | 14 (0.2) 10 (1) 1.2 (0.2) | 451 (5) 374 (11) 277 (23) | 43 (2) 29 (2) 26 (4) | 203 (2) 181 (5) 348 (39) |
| Auger solids Centrifuge solids AutoVacs | 2 2 7 | 259 (100) 247 (90) 261 (10) | 25,688 (8,563) 47,639 (17,341) 7852 (334) | 36 (8) 46 (16) 50 (2) | 38 (10) 82 (30) 40 (3) | 1,051 (249) 1,842 (607) 475 (15) | 247 (80) 293 (104) 1,595 (308) | 632 (166) 675 (181) 639 (58) | |
| 5500 cow | Press effluent Centrifuge effluent AutoVac effluent | 7 9 13 | 9.8 (0.3) 5.9 (0.3) 0.7 (0.1) | 989 (40) 693 (15) 215 (12) | 13 (1) 11 (0.3) 3.3 (1.7) | 8 (0.3) 7 (0.2) 2 (0.2) | 568 (23) 390 (9) 281 (13) | 25 (1) 17 (1) 75 (6) | 224 (9) 174 (3) 501 (26) |
| Centrifuge solids AutoVac solids | 10 12 | 97.4 (23) 242 (7) | 5,300 (307) 3,466 (231) | 45 (2) 65 (4) | 33 (1) 48 (3) | 3,124 (274) 871 (47) | 488 (38) 3,636 (204) | 1,007 (29) 1,014 (45) | |
[a]n = number of samples measured. | |||||||||
Other Nutrient Removal and Recovery
Most of the other nutrients measured, like phosphorus, were in particulate form and tended to be removed by the full MAPHEX System, with removal values ranging from 71% to 104% for Al, Ca, Mn, and Zn, with Mg having somewhat lower removal rates ranging from 47% to 60% (tables 3 and 4). Values shown here for Fe and S are partly a reflection of our adding Fe2(SO4)3 as a chemical treatment, but the data suggests that Fe was largely removed by the system, while S primarily remains in the effluent (tables 3 and 4). This finding is consistent with the intended chemical reactions which convert dissolved P into a particle: 1) disassociation of Fe2 and (SO4)3, 2) reaction of Fe with oxygen in water to form particulate ferric oxyhydroxides, and 3) sorption of dissolved P to the surface of the oxyhydroxides. Of special note is the case of the farm with the anaerobic digester. With the still anaerobic digestate, Fe remained in the liquid phase because the intended reaction of Fe with oxygen could not occur to completion, and so, much of the iron did not form particulate ferric oxyhydroxides. Overall, this data shows that the MAPHEX System is highly efficient at extracting and concentrating most nutrients in solid form.
MAPHEX Lite Design and Expected Performance
Armed with data from previous studies (Church et al., 2016, 2017, 2018, 2020) and the data presented here, we have designed, and now built, a simplified, but much larger System we call MAPHEX Lite. Our goals for the new System were to: (1) greatly increase the flow rate through the System so as to maximize the amount of manure treated per day, (2) remove a significant amount of P from dairy manures, (3) minimize the amount of N lost in the treatment process, (4) capable of operating around the clock with little human intervention, (5) avoid the use of costly chemicals and DE filtrate material, (6) reduce the amount of manpower necessary to run the System, and thus (7) reduce the overall cost of manure treatment. MAPHEX Lite consists of only the first two steps of the full MAPHEX System, bulk solids removal and medium solids removal (together comprising about 90% of total solids), and has the generator and all pumps and piping necessary fitted on a single semi-trailer (fig. 2). The new System will also be equipped with real-time solids sensors, and preliminary research indicates that we can use this data to develop a rating curve for real-time P removal. This will allow us to adjust flows to the screw press and centrifuge to fine tune P removal in real-time.
| Table 4. Percent removal of nutrients using MAPHEX system. | |||||||||||
| Percent Removal (%) | |||||||||||
| Dairy Farm | P | N | K | Al | Ca | Zn | Mn | Mg | Fe | S | |
| 90 cow | Full MAPHEX | 91 | 68 | 35 | 96 | 71 | 84 | 104 | 60 | 56 | 53 |
| 150 cow | Full MAPHEX | 92 | . | 22 | 96 | 85 | 87 | 99 | 52 | 59 | -46 |
| 2700 cow | Full MAPHEX MAPHEX Lite (Expected) | 88 37 | 58 20 | 34 15 | 95 59 | 93 61 | 85 27 | 93 42 | 47 28 | 56 51 | -49 22 |
| 5500 cow | Full MAPHEX MAPHEX Lite (Expected) | 86 47 | 39 . | -2 -13 | 95 59 | 83 47 | 80 32 | 81 34 | 55 38 | -122 51 | -48 49 |
 |
| Figure 2. The MAPHEX Lite System. |
Bulk Solids Removal
As discussed earlier, the slot size of the screw press (1 mm) compared to the two auger presses (3.2 and 4.8 mm) retained smaller particle sizes and was able to more efficiently remove P from manures. Furthermore, screw presses have much more common and robust use on farms that use manure solids for bedding than do auger presses. Therefore, we have chosen to use a screw press (Borger LLC, Chanhassen, Minn.) capable of operating at a rate of 475 L min-1. We will initially test it with a 1 mm slot size screen for bulk solids removal initially, and then test it with a 1.5-mm slot size screen if needed.
Medium Solids Removal
In the full MAPHEX System, the decanter centrifuge was highly efficient at removing and concentrating P and other nutrients, but this study has revealed that doing so is likely at the expense of losing N, especially in the high energy environment of the centrifuge. In addition to that, with some manures we had a foaming issue with the centrifuge effluent. We have therefore attempted to modify the way liquid is introduced and exits the centrifuge so as to lower effluent agitation in hopes to minimize both foaming and ammonia loss. We have also fitted the centrifuge internal auger with a computer-controlled drive motor. We expect that this will allow us some measure of control over the dryness of the centrifuge solids as, at some farms, we have had centrifuge solids with undesirable spreading characteristics, and will allow us to further fine tune P removal in real-time. In order to increase the volume of manure treated, we have fitted the new System with two identical centrifuges, each capable of 170 L min-1.
MAPHEX Lite Expected Performance
As we have tested equipment similar to what the new System will contain (screw press and two centrifuges) on the 2700- and 5550-cow dairies, we can project its likely performance (table 4). The combination of a screw press and centrifuge on these two dairies removed 37% and 47% of P, respectively. On other farms however, a similar combination has shown as high as 60% P removal (Church et al., 2016, 2017), so we anticipate that with fine tuning we will be able to achieve at least 50% P removal on all farms. N removal, including ammonia loss, was only 20% compared to the 58% for the full System on the 2700-cow dairy, indicating that, even without the modifications mentioned above, most of the N should be retained in the liquid phase for the beneficial use of the farmer. However, we plan to conduct future measurements to document loss of ammonia. Likewise, K removal for the new is projected to be quite low at 15% and -13% (again, negative due to either volume changes and very low K concentrations in the solids removed, or due to K amendment in the anaerobic digester) for the 2700- and 5500-cow dairies, respectively. Of the other nutrients reported here, removal for the new System is projected to be intermediate, ranging from a high of 61% for Ca on the farm that amended manure solids with lime, to a low of 22% removal for S.
MAPHEX Lite Cost Reductions
The new System is expected to greatly increase flow rates and reduce costs as well. The full MAPHEX System is limited to the maximum flow rate of the AutoVac® of 55 to 75 L min-1. The new System will be limited by the flow rates of the two centrifuges of 340 L min-1, resulting in a lower cost per liter of manure treated. Eliminating the chemical treatment and final filtration steps eliminates costs of the Fe2(SO4)3 ($0.88 per kg) and the DE Filtrate material ($0.73 per kg). It also reduces energy needed to run the System on a per liter basis and reduces operator time and operator knowledge required to run it. Based on experience from operating an identical centrifuge in the Full System and on horsepower ratings for the screw press, we project that the costs to run MAPHEX Lite will be less than $0.13 L-1, which compares favorably to an estimated cost of $0.39 L-1 for manure hauling. Furthermore, regarding the economics of a screw press and centrifuge combination, a recent modelling study (Rotz et al., 2022) highlighted that, not only does this combination allow the farmer more precision in application of P where it is needed, but can also be profitable for the farmer by recovering and concentrating P, thus reducing the amount of fuel, labor, and trucks needed to haul manure to remote fields that need P and away from those that have a surfeit of P.
Conclusion
Overall, the data presented here indicate that both the full MAPHEX System and the newly designed MAPHEX Lite System are highly efficient at extracting and concentrating most manure nutrients in solid form while leaving most of the N and K in the liquid form for beneficial use by the farmer near the manure source. Furthermore, the simplified system, being more economical while requiring less operator time and expertise, should make the technology more adoptable by farmers, especially when considering the rising cost of commercial fertilizers. Therefore, it seems clear that both Systems, and the components they include have the potential to play a significant role in manureshed management.
Acknowledgments
This work presented here is covered by Patent Application Number 14665229, filed 23 March 2015, and Patent Pending as of 28 August 2017. Special thanks are extended to David Otto (retired), who contributed significantly to the design of both the prototype and full-scale MAPHEX Systems. Funding for this study was provided by a Research Applications for INnovation (RAIN) Grant from the Pennsylvania State University Foundation, and the USDA-NRCS Conservation Effects Assessment Program.
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